Receiver muting circuit



ct. 26, 1948. M. G. CROSBY 2,452,435

l RECEIVER MUTING CIRCUIT Filed Juiy 1, 194.3 2 sheets-sheet 1 M. GCROSBY- RECEIVER MUTING CIRCUIT 0d. ze, 1948.

' 2 Sheets-Sheet 2 Filed July l, 1945 @TPV Patented Oct. 26, 1948 lorifice RECEVER MUTING CIRCUIT Murray Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application July 1, 1943, Serial No. 493,052 1 Claim. (Cl. Z50-20) My present invention relates to noise control for angle modulated carrier wave receiversgenerally, and more specicallyto automatic muting circuits for frequency modulation receivers.

One of the main objects of my present invention is to provide a simple and highly effective means for obtaining control overa receiver of angle modulated carrier waves so as to eliminate the rush of noise which usually occurs when the received modulated signal falls below a usable intensity level.

Another-important object of my invention is to prevent inter-station noise during tuning of a frequency modulationreceiver; the noise being prevented very simply, and without the addition of complicated circuits, by deriving a muting bias from the usual limiter output energy in response to the received carrier energy decreasing towards the point where the peak received carrier voltage is substantially equal to the peak noise voltage.

Another object of my invention is to provide a muting control for a frequency modulation receiver; the control method having the outstanding advantage that the control is independent of variations in gain of the amplifying circuits, and control voltage being derived from signal energy subsequent to the limiter output terminals.

Still other objects of my invention are to provide simpliiied muting arrangements for frequency modulation receivers, wherein a high level of control potential is provided which does not require amplification. t lThe generic expression angle modulated vcarrier wave is meant to include frequency modulated and phase modulated carrier wave energy. The abbreviation FM is used hereinafter to denote frequency modulation.

Still other objects of my invention are to improve generally the efliciency and reliability of FM receivers, and more especially to provide a noise-free FM receiver capable of being economically and simply manufactured and assembled.

The novel features which I believe to' be characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into eiiect.

In the.. drawing:

Fig. 1 schematically shows an FM receiver em-f bodying the invention, Fig. 2 shows the limiter characteristic at the limiter input terminals in idealized form, K

Fig. 3 shows an ideal muting characteristic of the invention,

Fig. 4 is a circuit diagram of a modiiication. Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements, the FM receiving system shown in Fig. 1 has its conventional networks represented schematically. Those skilled in the art of radio communication are fully aware of the manner of constructing and operating an FM receiver adapted to operate on the superheterodyne principle. While my invention is not restricted to use in a radio receivernor to any speciiic range of frequencies, by way of specic example let it be assumed that the receiver is adapted'to be operated in the present FM band of 42 to 50 megacycles-(mc). In this band each station is permitted an overall maximum frequency swing of kilocycles (kc.) .Y The `operating intermediate frequency (I. F.) may be chosen. from a range of 4 to 20 kc;4 An I. F.

value of 8.33 mc. is found to be a satisfactory compromise for .all factors.- V.

The FM wave energy is collected by an antenna, such as dipole I, and fed to la tunable radio frequency amplifier 2. The FM waves may be se# lected and ampli'lied in several cascaded ampliier stages, if desired. The'selector circuits, of

course, are designed to pass the entire frequency swing of the waves; hence, they are given passe band widths of at least 150 kc. The converter 3 acts to reduce the center or carrier frequency of the amplied FM waves to a lower I, F. value without changing theirequency swing. The LF. energy may be amplified in one or more amplifier stages, as indicated at il. The lusual amplitude modulation limiter 5 is employed greatly to reduce any amplitude modulation eiiects which may have developed on the FM waves in their passage' to the input terminals of the limiter. The limiter may be of any known type. lts input-output characteristic is shown above the rectangle 5.'

Generally speaking, the limiter is usually a read-A ily-saturatable ampliiier which provides a sub-` stantially flat output above a predetermined signal input level. Bothv grid circuit and plate circuit limiting action may be employed in the lim-` iter tube. l l r The limited FM wave energy, with a mean ire quency Fc at the operating I. vF. value, is applied tothe FM detector circuit 6. Any well-known 3` and Suitable circuit having the inclined S -shaped characteristic, shown above rectangle 6, may be employed. As is well known, it is common practice to have a pass-band Width in excess of the 150 kc. swing at the detector input circuit. The usual FM detector acts to translate the FM carrier Wave energy into corresponding AM carrier wave energy, thelatter being rectied to provide the modulation signals originally used to modulate the carrier at the transmitter station. The FM wave energy supplied to the detector 6 is a frequency-variable wave. That is, the freduen'y swings or deviations of the mean frequency Fc correspond to amplitude of` the `modulation sig nals, While the modulation frequencies are repre'- sented by the rate of frequency deviation.

" output energy in any Well-known manner. For

The concurrent discrimination and Vrectiiican4 tion occurs at 6 to provide the audio modulation,

if a predetermined signal to noise ratio exists' at the input terminals of the limiter. When the ratio falls below a .certain threshold value (hereinafter termed improvement threshold) the audio output ofI audio amplifier l is essentially noise voltage. The latter, When reproduced by the reproducer, is very disturbing to listeners. This noisy reception Will usually occur during tuning between stations, and causes the familiar inter-station rush of noise in an FM receiver. It will, also, occur as a result of deep fading, since in this case the signal to noise ratio becomes very low.

I have found that the above noise may be eliminated very effectively after a consideration of the following phenomena: A limiter is not capable of removing amplitude modulation hav ing a depth of modulation as greatas 100%. This is true, since in the condition of l100% modulation the amplitude ci the carrier `vvave is zero for certain intervals. Hence, to provide complete limiting the limiter must supplya signal gain which raises the signal level from zeroto a finite value. This is, of course, impossible. Consequently, when the condition of 1,00% modulation occurs at the limiter input, theoutputenergy is amplitude modulated regardless of the degree of limiting obtainable by the limiter. The condition of 100% amplitude modulation in the FM Wave energy applied to the limiter occurs at the improvement threshold. This threshold point is depicted in Fig. 2 as the rather abrupt drop inthe idealized curve which relates signal to noise ratio as ordinates to peak noise voltage/peak .carrier voltageV'as abscissae. The curve in Fig. 2 is purely illustrative. The improvement threshold point may be defined as the point of equality' of the peakl voltage of the noise and that of the incoming carrier. Reference is made to my paper frequency modulation' noise characteristics in the April 1937 issue of the Proceedings of the Institute of Radio Engineers, for a more complete analysis of the improvement threshold character istics.

At the aforesaid point oi equality there is a complete cancellation of the signal by the noise. This produces intervals of `zero signal. As ex-` plained above, such intervals are characteristic of 100% amplitude modulation. They output energy of the limiter rather suddenly becomes amplitude modulated by the noise at the improvement threshold. Accordingly, any rectiiier device Will automatically provide a voltage representative of the amplitude modulation at the threshold point, In Fig. 3 I have shown graphically the ideal manner in which control voltage would example, a simple diode rectiiier 8, with a resonant inputcircuit 9 tuned to the mean frequency Fe, may have the `limiter output energy applied thereto. There will be developed across the rectifier load resistor l0, properly bypassed for highl frequency currents, a rectifier voltage corresponding to amplitude modulation existing in the limiter output energy.

The rectiiier output voltage, which is actually detected noise components extending up to superaudible frequencies, is fed to an additional rectiiier ll for conversion to direct current voltage.

. If a very high level of control voltage is desired,

ampliiication may be provided prior to rectiiier H. `The rectifier Il is shown as a diode Whose resistor load l2 has its ungrounded end coupled to the ungrounded'end of resistor l0 by con-- denser i3. The rectified voltage across resistor I2 is transmitted as a muting voltage over lead i4. A lter R, C removes any pulsating voltage components.

The anode end of resistorlz assumes a voltage 'i' Which is negative relative to ground upon the appearance of amplitude modulation in the limiter output energy. This negative voltage is applied through the 'grid resistor to the input grid'of the audio amplifier tube (not shown). As a result the amplifier will be biased to cut-oli thereby muting or squelching any noise reproduc-v tion. It is not necessary to produce squelch action by negative biasing to cut-oil?, Any otherwellknown form of noise muting circuit may be used to silence the receiver in response to control voltage derived from the limiter output energy atlthe improvement threshold.

It is notiessential to derive the muting voltage from the limiter output terminals directly. Nor is it necessary to use a separate rectiiier to rectift the amplitude modulation existing in the limiter output energy below the improvement threshold. In Fig. 4 Ihave shown a modification of my inventionwherein the control voltage is derived right at the FM detector circuit in response to a decrease of signalto noise ratio below the improvement threshold. In this modiiication the FM detector circuit is of the type disclosed and claimed in my U. S. Patent No. 2,296,092 granted September 15th, 1942. As a result the gain of tube 'I Will be reducedto the point Where audio reproduction is prevented. The operation of this type of detector is fully described in the said patent; it will be briefly referred to herein.

- There are employed a pair of opposed diode rectiers 20 and 2l. The diode input circuit 22 is coupled to the limiter output circuit 23. The input circuit 24 is coupled to output circuit 23. The latter is substantially tuned to Fc the operating I. F. value), While tuned circuits 22 and 2B are oppositely mistuned by equal values relative to'Fc. In this way discrimination is provided. The cathodes of diodes 20 and 12| are connected by series resistors 25 and 26, the cathode' end of resistor 26 being grounded. Resistor 21 is arranged in series with resistor 25, both resistors being bypassed for high frequencies. Resistor 28 is arranged in series with resistor 26, these resistors being bypassed as well. The useful audio modulation is taken off from the cathode end of resistor 25, and transmitted by condenser 30 to the control grid of audio amplier tube 1. A potentiometer l5 regulates the intensity of audio signal voltage applied to audio amplifier 1.

Any amplitude modulation on the carrier will be detected, and the audio output thereof will appear at the anode end of resistor 21. It will be seen that the load resistors 25 and 26 between the cathodes of rectiers 20 and 2| are such that they are differentially connected for FM wave energy, While resistors 21 and 26 are in aiding phase for amplitude modulation. The rectified voltages developed across resistors 25 and 26 are combined in polarity opposition relative to ground. The differential potential at the cathode end of resistor 25 varies in polarity and magnitude in accordance With the direction and degree of frequency variation of the FM signals at the input circuit 23. However, amplitude variations of the FM signals at circuit 23 cause equal rectified voltages across resistors 21 and 26, and these rectified voltages are combined in like phase relative to ground. Hence, upon the signal to noise ratio at the limiter input terminals decreasing to the improvement threshold, amplitude modulation will appear on the carrier energy at the limiter output terminals. Simultaneously, noise voltage will be fed through condenser i3 to the rectifier Il. The resulting direct current voltage across resistor l2 will be applied through lter R, C to lead I4. The negative muting bias Will be applied to the grid of tube 'l through the adjustable tap of potentiometer I5.

While I have indicated and described several systems for carrying my invention into eiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

6 What I claim is: In combination with a pair of opposed diode v rectifiers having a common frequency discrimivnator input circuit, each of said rectiers having a pair of series-connected load resistors across which rectified signal voltages are developed in response to frequency variations of signal energy at said input circuit, a common output circuit including a predetermined one of each said pair of said resistors in series relation, an amplifier having an input connection to a point of said output circuit at which said rectified voltages are combined in polarity opposition, a third diode rectifier having an input connection, said input connection being made to a point on one of the rectiiers to apply to the third rectier the total rectified voltages of like polarity across the second load resistor of said one rectifier and across the said predetermined one resistor of the second rectifier whereby voltage representative of amplitude modulation of said signal energy is applied to said third rectifier in response to the signal energy to noise energy ratio decreasing to a predetermined magnitude, and means responsive to rectied voltage derived from the third rectifier for rendering said amplier ineffective.

MURRAY G. CROSBY.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,152,515 Wheeler Mar. 28, 1939 2,261,643 Brown Nov. 4, 1941 2,263,633 Koch Nov. 25, 1941 2,264,019 Case Nov. 25, 1941 2,273,098 Foster Feb. 17, 1942 2,296,101 Foster Sept. 15, 1942 2,301,620 Fowler Nov. 10, 1942 2,301,649 Thompson Nov. 10, 1942 2,316,902 Trevor Apr. 20, 1943 2,400,948 Peterson May 28, 1946 2,404,338 Worcester, Jr July 16, 1946 

